1. Field of the Invention
The present invention relates to an apparatus that implements drive control on an AC motor and, more specifically, relates to an apparatus that reduces the higher harmonic current flowing to a 3-phase AC motor.
2. Description of the Related Art
The control calculation is normally executed in a current control circuit for a 3-phase AC motor by converting the physical quantity of a 3-phase AC which necessitates a complicated handling procedure to a direct current physical quantity in order to facilitate the calculation (see Japanese Laid-Open Patent Publication No. H 08-331885).
FIG. 14 shows the structure of a standard 3-phase AC motor control apparatus which is commonly utilized. In the control calculation executed in this motor control apparatus, a rotating orthogonal coordinate system (a dq coordinate system) having a d-axis representing the direction of the magnetic flux in the 3-phase AC motor and a q-axis extending perpendicular to the d-axis is used. The current control calculation is performed using a direct current value obtained by converting the 3-phase AC value in the rotating orthogonal coordinate system to reduce the current control deviation.
Miniaturization and higher efficiency are often achieved with regard to an AC motor today by providing a rotor having internally embedded magnets, as shown in FIG. 15, and a stator assuming a concentrated winding structure. The rotor is able to effectively utilize the magnetic torque and the reluctance torque. A motor having a rotor adopting this structure is called an IPM (interior permanent magnet motor). The stator is capable of greatly reducing the coil end. A motor provided with the rotor and the stator having the structures described above, which is called a concentrated winding IPM motor, has been attracting a great deal of interest as a motor capable of realizing miniaturization and a high order of efficiency.
While the concentrated winding IPM motor described above allows miniaturization and achieves higher efficiency, it is characterized in that significant spatial harmonics manifest. The spatial harmonics in a motor assuming a concentrated winding structure become significant because the small number of slots at each pole results in a more uneven distribution of the magnetic flux compared to that in a motor assuming a distributed winding structure. An explanation is given below as to why a uniform magnetic flux distribution cannot be achieved.
FIG. 16 shows an SPM motor assuming a surface magnet structure achieved by covering the surface of the rotor with magnets. Unlike in the SPM motor shown in FIG. 16, areas where magnets are embedded and areas where no magnet is embedded are present along the circumference of the rotor in the IPM motor with the internally embedded magnets shown in FIG. 15. Accordingly, while a uniform magnetic flux distribution is achieved in the SPM motor having a rotor, the surface of which is covered with magnets, a greater degree of change in the magnetic flux manifests in the IPM motor to result in a greater spatial harmonic component.
Greater spatial harmonics in the motor leads to a greater higher harmonic component in a current flowing to the motor, and thus, problems arise in that the extent to which the motor efficiency is improved is compromised and in that the extent of torque ripple becomes significant. In addition, since the higher harmonic component is superimposed on the fundamental wave component in the current, another problem manifests in that the current peak value increases.
Addressing these problems, the applicant of the present invention proposed a motor control apparatus that reduces the higher harmonics current flowing to an AC motor in Japanese Patent Application No. 2000-356117. This motor control apparatus includes a fundamental wave current control system that controls a motor current in a dq-axis coordinate system which rotates in synchronization with the motor rotation rate and a higher harmonic current control system that controls the motor current in an orthogonal coordinate system rotating at a rotation rate which is an integral multiple of the rotation rate of the dq-axis coordinate system, and generates a voltage command value for a voltage to be applied to the motor based upon the outputs from the two current control systems. The motor control apparatus greatly reduces the higher harmonic current component.
However, while the motor control apparatus (Japanese Patent Application No. 2000-356117) which includes the two current control systems, i.e., the fundamental wave current control system primarily for controlling the fundamental wave current component and the higher harmonic current control system primarily for controlling the higher harmonic current component achieves control performance in controlling the higher harmonic current component that is greatly improved over the control performance of motor control apparatuses in the related art under normal circumstances, a problem arises when the voltage becomes saturated in that the resulting current distortion is more pronounced than in the motor control apparatuses in the related art.
Normally, a PWM invertor is utilized to implement drive control on an AC motor. A PWM invertor converts a DC voltage supplied from a DC source to an AC voltage equivalent to a voltage command value through the PWM and applies the AC voltage to the motor. Thus, the level of the DC voltage from the DC source determines the maximum value of the AC voltage that can be output.
In the operating range of a motor capable of a large output, the level of the voltage to be applied to the motor is close to the maximum voltage that can be output by the invertor. In addition, it is necessary to apply a large transient voltage to the motor in order to rapidly increase the output, as well. While a voltage, the level of which is higher than that of the maximum voltage that the invertor is capable of outputting may be required under these circumstances, a voltage higher than the maximum that can be output by the invertor cannot be applied to the motor. Hereafter, the state in which a voltage higher than the voltage that the invertor is capable of outputting needs to be applied to the motor is referred to as a xe2x80x9cvoltage-saturatedxe2x80x9d state. A voltage saturation tends to occur readily in the motor control apparatus described above in which the higher harmonic component in the voltage increases as a result of the reduction in the higher harmonic current component.
The required current can no longer be supplied to the motor under these circumstances due to an insufficient voltage output. However, the output voltage command value is continuously changed along the direction in which the necessary current is supplied through the two current control systems, i.e., the fundamental wave current control system and the higher harmonic current control system. As a result, a serious current distortion occurs, for instance, after the operation recovers from the voltage-saturated state.
An object of the present invention is to reduce the extent of a current distortion resulting from saturation of an output voltage in a motor control apparatus having a fundamental wave current control system and a higher harmonic current control system.
The motor control apparatus according to the present invention comprises a fundamental wave current control system that controls a fundamental harmonic wave component of a motor current in an orthogonal coordinate system which hypothetically rotates in synchronization with a rotation of a 3-phase AC motor, a higher harmonic current control system that controls a higher harmonic component contained in the motor current in an orthogonal coordinate system hypothetically rotating at a frequency which is an integral multiple of the frequency of the fundamental wave component of the motor current, a voltage command value generating device that generates a 3-phase AC voltage command value by adding the output from the fundamental wave current control system and the output from the higher harmonic current control system, a power conversion device that converts a DC source voltage to a 3-phase AC voltage corresponding to the 3-phase AC voltage command value and outputs the 3-phase AC voltage to the 3-phase AC motor, a voltage saturation detection device which detects that an output voltage from the power conversion device is in a saturated state and a gain adjustment device that reduces a current control gain for the higher harmonic current control system if the voltage saturation detection device detects that the output voltage is in a saturated state.
In the motor control method adopted in the control system which includes a fundamental wave current control system that controls a fundamental wave component of the motor current in an orthogonal coordinate system hypothetically rotating in synchronization with the rotation of a 3-phase AC motor and a higher harmonic current control system that controls a higher harmonic component contained in the motor current in an orthogonal coordinate system hypothetically rotating at a frequency which is an integral multiple of the frequency of the fundamental wave component of the motor current and achieved by adding the output from the fundamental wave current control system and the output from the higher harmonic current control system to generate a 3-phase AC voltage command value, converting a DC source to a 3-phase AC voltage corresponding to the 3-phase AC voltage command value and outputting the 3-phase AC voltage to the 3-phase AC motor, a detection is performed to ascertain whether or not the output voltage output to the 3-phase AC motor by converting the DC source voltage to the 3-phase AC voltage is in a saturated state and the current control gain for the higher harmonic current control system is reduced if the output voltage is in a saturated state.